Glossary

Absolute dating the process of determining a specific date (in years or some other unit of time) for an archaeological, geological or paleontological site or artifact. (Wikipedia glossary)

Absolute time Geologic time expressed in years before the present.(S.M. Richardson)

Abundant metal Iron, aluminum, magnesium, manganese, and titanium. Ores of the abundant metals only need to be 3 - 5 times as metal-rich as average rock.

Abyssal plain Large area of extremely flat ocean floor lying near a continent and generally over 4 km in depth. (S.M. Richardson)

Accretion (a) The process by which the terrestrial planets grew, increasing their mass by gradually accumulating smaller bodies, called planetesimals. (S.M. Richardson). (b) a process by which material is added to a tectonic plate. (Wikipedia glossary)

Active layer The seasonally thawed zone above permafrost . (S.M. Richardson)

Alluvial fan Land counterpart of a delta . An assemblage of sediments marking place where a stream moves from a steep gradient to a flatter gradient and suddenly loses transporting power. Typical of arid and semiarid climates but not confined to them. (S.M. Richardson)

Alluvium A general term for clay, silt, sand, gravel or similar unconsolidated material deposited by a stream or other body of running water. (GeoMan).

Angular unconformity an unconformity in which younger strata overlie an erosion surface on tilted of folded layered rock. (Wikipedia glossary). So called because the beds below the unconformity dip at a different angle than the beds above it. (S.M. Richardson)

Anticline A fold that is convex upward, or that had such an attitude at some stage of its development. compare syncline. (S.M. Richardson)

Aphanitic refers to the texture of igneous rocks when you can't distinguish individual minerals unless you use a microscope then it is said to have an aphanitic texture. (H. Redoble, Wikiversity)

Asthenosphere the layer of the mantle that lies directly below the lithosphere. (A. Senos, Wikiversity)

Archean An eon of geologic time extending from about 3.8 to 2.5 billion years ago. It was preceded by the Hadean and followed by the Proterozoic.

Banded Iron Formation Rock consisting of alternating light and dark layers of iron-rich chert (the dark layers have more iron minerals) formed from 3.8 to 1.7 billion years ago. (UCMP)

Basalt Volcanic rock (or magma) that is generally dark in color, contains 45 to 54 percent silica, and is rich in iron and magnesium. An eruption of basaltic magma is generally quiet, and results in flows (both vesicular and non-vesicular) and breccias. Undersea eruptions commonly result in the formation of "pillow lavas." Basalt represents the initial differentiated material erupted by the earth at spreading centers, and is considered by GeoMan to be the "blood of the earth." (GeoMan)

Biostratigraphy branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. (Wikipedia glossary)

Carbonate n. (adj.) A mineral composed mainly of calcium (Ca²⁺) and carbonate (CO₃^2-) ions, may also include magnesium (Mg), iron (Fe) and others; n. rock or sediments derived from debris of organic materials composed mainly of calcium and carbonate (e.g., shells, corals, etc.) or from the inorganic precipitation of calcium (and other ions) and carbonate from solution (seawater). For example, limestone or dolomite. carbonate platform – n. A broad (100s of meters), flat, shallow submarine expanse of carbonate rock, more common in the early-middle Paleozoic. carbonate bank – n. A narrow (10s of meters), fairly flat, shallow, submarine plateau of carbonate rock, more common from the middle-late Paleozoic to the present, e.g., the Bahama Banks. (UCMP)

Caldera The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades. (GeoMan)

Chalk A soft compact calcite, CaCO₃, with varying amounts of silica, quartz, feldspar, or other mineral impurities, generally gray-white or yellow-white and derived chiefly from fossil seashells. (UCMP)

Chert Hard, dense sedimentary rock, composed of interlocking quartz crystals and possibly amorphous silica (opal). The origin of the silica is normally biological, from diatoms, radiolaria or sponge spicules. Synonymous with flint. (UCMP)

Cinder cone A volcanic cone built entirely of loose fragmented material (pyroclastics). (GeoMan)

Cleavage When a mineral is broken, cleavage is shown by the pattern in which it breaks along crystallographic planes. If it does not follow these planes then it is considered a fracture. (T. Bakic) (Wikiversity)

Coastal plain A low plain of little relief adjacent to the ocean and covered with gently dipping sediments. (Geotech org)

Composite volcano A steep volcanic cone built by both lava flows and pyroclastic eruptions. (GeoMan)

Continental arc A belt of volcanic mountains on the continental mainland that lie above a subduction zone. compare island arc. (S.M. Richardson)

Continental crust The part of the crust that directly underlies the continents and continental shelves. Averages about 35 km in thickness, but may be over 70 km thick under largest mountain ranges. (S.M. Richardson)

Continental deserts Located in continental interior far from moisture-bearing winds. (S.M. Richardson)

Continental divide A major drainage divide separating the drainage to one ocean from another. (S.M. Richardson)

Continental drift the movement of the Earth's continents relative to each other. The hypothesis was proposed by Alfred Wegener in 1912, to explain the relative positions and shapes of continents, the formation of mountains, and other large-scale geologic phenomena as results of the lateral movement of continents. The crust of ocean basins was assumed to be relatively immobile. It was not until an understanding of plate tectonics in the 1960s, that an actual geological explanation of that movement was found. There is considerable paleontological and geological evidence in favour of continental drift. See also plate tectonics, sea floor spreading. (based on S.M. Richardson and Wikipedia)

Continental rise Part of the continental margin; the ocean floor from the continental slope to the abyssal plain. The continental rise generally has a gentle slope and smooth topography. (UCMP)

Continental shelf The part of the continental margin from the coastal shore to the continental slope; usually extending to a depth of about 200 meters and with a very slight slope, roughly 0.1 degrees; includes continental and oceanic sediments down to the ocean floor. (UCMP)

Continental slope n. Part of the continental margin; the ocean floor from the continental shelf to the continental rise or oceanic trench. Usually to a depth of about 200 meters. The continental slope typically has a relatively steep grade, from 3 to 6 degrees. (UCMP)

Continental volcanic arch mountain ranges produced by igneous activity due to the subduction of the ocean's lithosphere. (Wikiversity)

Core A cylindrical section of rock, usually 2-4 inches in diameter and up to several feet long, that is the result of coring into the earth. Individual cores are brought to the surface for geologic examination and/or laboratory analysis. (USGS Paleontology glossary)

Craton The stable portions of the continents that have escaped orogenic activity for the last 2 billion years. Made predominantly of granite and metamorphic rocks. compare orogen. (S.M. Richardson)

Creep 1. The very slow, generally continuous downslope movement of soil and debris under the influence of gravity. 2. The movement of sand grains along the land surface.(S.M. Richardson)

Curie Point The temperature a substance reaches when its magnetism is lost. (J. Mellum, Wikiversity)

Debris avalanche A flow of unsorted masses of rock and other material downslope under the influence of gravity. Water is commonly involved as a catalyst and/or lubricant. For example: a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacial ice, trees and other debris, and hot pyroclastic material was associated with the May 18, 1980 eruption of Mt. St. Helens. Most of the deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche resulting from the eruption. (GeoMan)

Deep time Time measured in terms of geological processes (erosion, mountain building, glaciation, etc), or measured in terms of that involve millions of years, as opposed to mere decades or centuries as in human history. ; geological or cosmological time. The work of 18th and 19th century geologists such as Hutton and Lyell showed that geological processes occurred over vast periods of time, overturned biblical theology which thought in terms of mere centuries or millennia, and made possible evolutionary science, and our understanding of the Earth and the universe. Palaeos is all about Deep Time. (MAK)

Delta An assemblage of sediments accumulated where a stream flows into a body of standing water and its velocity and transporting power are suddenly reduced. . A "delta plain" is the upper surface of a delta. (S.M. Richardson)

Diagenesis All chemical, physical, and biological modifications undergone by sediments from the time of their initial deposition, through their conversion to solid rock, and subsequently to the brink of metamorphism. (S.M. Richardson, USGS Paleontology glossary)

Dome (1) A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent, often circular in plane view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. (GeoMan) (2) An uplift or anticlinal structure, roughly circular in its outcrop exposure, in which beds dip gently away from the center in all directions. (S.M. Richardson)

Dormant volcano This term is used to describe a volcano which is presently inactive but which may erupt again. The major volcanic cones of the Cascade Mountains (in Washington, Oregon, and California) are believed to be dormant rather than extinct. (GeoMan)

Drainage basin The area from which a stream and its tributaries receives its water. (S.M. Richardson) A region of land surrounded by divides and crossed by streams that eventually converge to one river or lake. (Geotech org)

Drift (glacial) General term for material deposited by a glacier. (GeoMan)

Earthquake The violent oscillatory motion of the ground caused by the passage of seismic waves radiating from a fault along which sudden movement has taken place. (Geotech org)

Ejecta Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes, lava bombs. (GeoMan)

Elasticity The tendency for a body to return to its original shape and size when a stress is removed. (S.M. Richardson)

Eolian Pertaining to or deposited by wind. (Geotech org)

Eon The largest division of geologic time, embracing several Eras, for example, the Phanerozoic, 600 m.y. ago to present); also any span of one billion years. (Geotech org)

Era The second largest division of geologic time, including several periods, for example, the Mesozoic era consists of the Triassic, Jurassic, and Cretaceous periods. Eras are grouped into eons. (MAK)

Erosion The transportation of material by a mobile agent, i.e., water, wind or ice. (C. Simoneau, Wikiversity)

Erratic (glacial) Large rocks or other debris deposited by a glacier, usually in an area far removed from its source. Commonly used to indicate a big chunk of debris which is clearly out of place and shouldn't even be where it is. (GeoMan)

Estuary An area where fresh water comes into contact with seawater, usually in a partly enclosed coastal body of water; a mix of fresh and salt water where the current of a stream meets the tides; hence estuarine. (UCMP)

Eugeosyncline The seaward part of a geosyncline; characterized by clastic sediments and volcanism. (Geotech org)

Eustatic change Sea level changes that affect the whole Earth. (Geotech org)

Evaporite Sedimentary rocks made up of minerals that are deposited from the precipitation of evaporated seawater. (M. MacFadzen, Wikiversity)

Fault A crack or fracture in the earth's surface in which there has been movement of one or both sides relative to the other. Movement along the fault can cause earthquakes or, in the process of mountain-building, can release underlying magma and permit it to rise to the surface as a volcanic eruption. (GeoMan)

Fault zone zone where exist different discrete fault planes. (Wikipedia glossary)

Fossil mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. (Wikipedia glossary); Evidence of past life on earth. Can include the preserved hard and soft parts of plants and animals, tracks and burrows, whole organisms preserved intact in amber or tar, and fossilized dung. Any evidence of life constitutes a fossil. (GeoMan)

Floodplain The low relief lands bordering a stream or river, common to the mature and old age stages of stream development. Floodplains store excess water in times of high water, and excess sediments in times of low water. Beware of building your dream house on a floodplain - they tend to get rather wet at irregular intervals. (GeoMan)

Fracture when a mineral breaks and does not cleave the mineral fractures. Fractures can be irregular, curved, jagged or splintery. Quartz actually has a conchoidal fracture which is dish shaped. (H. Redoble, Wikiversity)

Geochemistry The science that deals with chemical changes in and composition of the earth's crust. (USGS Paleontology glossary)

Geological time The period of time extending from the formation of the earth to the present. (USGS Paleontology glossary) Also may refer to the Geologic Time Scale.

Geologic(al) timescale An arbitrary chronologic sequence of geologic events, used as a measure of the age of any part of geologic time, usually presented in the form of a chart showing the names of the various rock-stratigraphic, time-stratigraphic, or geologic-time units. (USGS Paleontology glossary) It constitutes the standardised system of chronological measurement and dating, first developed during the early 19th century and revised ever since, which relates stratigraphy to time. It is used by geologists, paleontologists and other scientists to describe the timing of events that occurred during the history of the Earth, for example the age of particular rock strata, mountain building, or evolutionary radiation. The entire geological timescale consists of four eons, each divided into a number of eras. Each era is in turn divided into periods. Periods are further divided into epochs, which in turn are divided into ages. The regulatory scientific body that deals with this subject, the International Commission on Stratigraphy, uses an arbitrary colour code which nevertheless allows us to make Palaeos very colourful. The basic geological table is shown below; in keeping with stratigraphic and geological formalism, the oldest eras are at the bottom, the youngest at the top.

Geology The science that deals with the study of the planet Earth--the materials of which it is made, the processes that act to change these materials from one form to another, and the history recorded by these materials; the forces acting to deform the outer layers of the Earth and create ocean basins and continents; the processes that modify the Earth's surface; the application of geologic knowledge to the search for useful materials and the understanding of the relationship of geologic processes to people. (S.M. Richardson) Physical Geology includes processes that affect the earth's internal and external structure, composition and other natural functions. Historical Geology is the study of Earth history and the evolution of life on earth, especially the past life forms that are preserved as fossils. (J. Wittstrom, Wikiversity)

Geosyncline A downwarping of the Earth's crust, either elongate or basin-like, measured in scores of kilometers, in which sedimentary and volcanic rocks accumulate to thicknesses of thousands of meters. Not in current use since the development of plate tectonic theory. (S.M. Richardson)

Glacial abrasion A common mechanical weathering process where rock and debris frozen into the sides and bottom of a glacier act like sandpaper and wear down the bedrock the glacier is moving across. (GeoMan)

Glacial ice; glacier ice Ice with interlocking crystals that makes up the bulk of a glacier. (S.M. Richardson) Naturally occurring ice which exhibits internal plastic flow and deformation. (GeoMan)

Glacial quarrying (plucking) A common mechanical weathering process in alpine glaciated terrain where glacial ice frozen into cracks in the bedrock literally "pluck" rock material from the valley floor. (GeoMan)

Glacial polish Polished bedrock surfaces left behind after melting of glacial ice. The polishing is probably due to very fine grained rock flour carried at the base of the ice. (GeoMan)

Glaciation The formation, advance and retreat of glaciers and the results of these activities. (S.M. Richardson)

Glacier A mass of ice, formed by the recrystallization of snow, that flows forward, or has flowed at some time in the past. (S.M. Richardson)

Gneiss A coarse, foliated metamorphic rock in which bands of granular minerals (commonly quartz and feldspars) alternate with bands of flaky or elongate minerals (e.g., micas, pyroxenes). Generally less than 50% of the minerals are aligned in a parallel orientation. (S.M. Richardson)

Gondwana The southern land mass derived from the supercontinent of Pangea, which continued until its break-up during the Cretaceous and early Tertiary. It comprised of Antarctica, Africa, South America, Australia and India. The term is also used to describe these same continents when connected as a supercontinent in the Paleozoic, prior to Pangea. Gondwana means "Land of the Gonds" (a tribe from the Indian subcontinent). Note, the popular term Gondwanaland is therefore redundant.

Graben An elongate crustal block that is relatively depressed (downdropped) between two fault systems. (GeoMan)

Granite A coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodic plagioclase feldspar, and micas. Also sometimes a metamorphic product. (Geotech org)

Greenstone belt zone of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from the green hue imparted by the colour of the metamorphic minerals within the mafic rocks. Chlorite, actinolite and other green amphiboles are the typical green minerals. Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes. Greenstone belts are primarily formed of volcanic rocks, dominated by basalt, with minor sedimentary rocks inter-leaving the volcanic formations. Through time, the degree of sediment contained within greenstone belts has risen, and the amount of ultramafic rock (either as layered intrusions or as volcanic komatiite) has decreased. (Wikipedia)

Halide a binary compound constructed of two parts, one halogen atom and one element or radical that has a electronegative value less than the halogen. (Q. Janmohamed, Wikiversity)

Hardness A property of a mineral. Its tolerance or resistance to scratching or abrasion. This is measured by the Mohs scale. (Christine Simoneau, Wikiversity)

Harry Hess In the 1960s Hess developed the theory that the seafloor is spreading. This theory was based on the discovery of the Mid-Atlantic Ridge, proving that the upwelling of magma breaking through the mantle is pushing the seafloor from the ridge crest. (J. Howley, Wikiversity)

Hiatus A gap or interruption in the continuity of the geologic record either because the record was never formed or because it was destroyed by erosion. It represents the time interval spanned by an unconformity. (S.M. Richardson)

Horizon in this context, a specific and distinctive period or layer of soil, rock strata, etc. (MAK)

Hot Spot A stationary mantle plume which will form a series of volcanoes or sea mountains as the earth's crust moves over it. (M. Laery, Wikiversity)

Hydrogeology The science that deals with subsurface waters and geologic aspects of surface waters. (USGS Paleontology glossary)

Hypersaline Extremely salty, having much more salt than normal seawater. (UCMP)

Igneous rock A rock that has crystallized from a molten state. (S.M. Richardson) Rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. (Wikipedia glossary)

Intermontane basin A basin between mountain ranges, often formed over a graben. (Geotech org)

Intrusion An igneous rock body that has forced its way in a molten state into surrounding country rock. (Geotech org)

Intrusive rock Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations. (Geotech org)

Island arc n. A curved chain of islands that rise from the sea floor, usually near a continent. The convex side usually faces the open ocean, while the concave side usually faces the continent, e.g., the Aleutian Islands in Alaska; volcanic arc (UCMP); chain of volcanic islands or mountains formed by plate tectonics as an oceanic tectonic plate subducts under another tectonic plate and produces magma. (Wikipedia glossary)

Langley The unit of solar energy relating to the amount which reaches a specific area of the earth's surface. In general, more "langleys" reach the surface of the earth at the equator than at the poles. (GeoMan)

Laurasia the Mesozoic northern supercontinent that was derived from Pangea. It comprised of North America, Eurasia (exclusive of India) and Greenland. Most of the famous Cretaceous dinosaurs are from Laurasia; Laurasia's sister continent Gondwana being inhabited by different types of animals. (Glossary - Bristol University; MAK)

Limestone the most abundant of the non-clastic sedimentary rocks that is produced from the mineral calcite (calcium carbonate) and sediment. The main source of limestone is the limy ooze formed in the ocean. The calcium carbonate can be precipitated from ocean water or it can be formed from sea creatures that secrete lime such as algae and coral. (Fossil Mall)

Lithosphere located above the asthenosphere, the lithosphere consists of the outer layer of earth's crust and upper mantle. The lithosphere also contains the Moho boundary. (H.Redoble, Wikiversity)

Lithostratigraphic or Lithologic Unit body of rock that is consistently dominated by a certain lithology or similar color, mineralogic composition, and grain size. It may be igneous, sedimentary, or metamorphic and may or may not be consolidated. (USGS Paleontology glossary)

Lower In geology, refers to lower, older strata. In the geological timescale, refers to the older rocks of a period, especially when the period is divided into three; e.g. "Lower Jurassic". This usage is now generally replaced by the more chronological descriptive term Early. (MAK)

Lustre the reflective quality of light on the freshly broken surface of a mineral. The main divisions of lustre are metallic and non-metallic. (M. MacFadzen, Wikiversity)

Mafic mineral A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine. (Geotech org)

Magma A body of molten rock found at depth. (C Simoneau, Wikiversity)

Magma chamber The subterranean cavity containing magma. When a conduit is opened to the surface, a volcanic eruption is possible. (GeoMan)

Magnitude A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs.) The number scale for magnitude is a modified logarithmic value, rather than arithmetic, and the numbers get real big, real fast; a magnitude 9 earthquake, for example, is 33 times greater than a magnitude 8 earthquake, 1089 times greater than a magnitude 7 earthquake, 35937 times greater than a magnitude 6 earthquake, and so on. The short version? Small quakes don't really do much to relieve stress in the crust. (GeoMan)

Marker horizon (or bed) A distinctive horizon which is used for regional correlation of lithology. A good marker horizon is distinctive, widespread, and represents a relatively short period of geologic time. For example, ash from a volcanic eruption, debris from a meteorite impact, etc. It is GeoMan's opinion that humans will represent one of the earth's finest marker horizons in the geologic record of the future. Our effect on the surface is certainly distinctive and widespread, and, at the rate we are going, it is likely that our species will have a relatively short lifespan (speaking in terms of geologic time, of course). (GeoMan)

Magnetometer Am instrument used for measuring a magnetic field's magnitude and direction. (T. Bakic, Wikiversity)

Mantle highly viscous layer directly under the crust, and above the outer core. (Wikipedia glossary)

Mesosphere The lower mantle. (Geotech org)

Metamorphic From the Greek "meta" (change) and "morph" (form). Commonly occurs to rocks which are subjected to increased heat and/or pressure. Also applies to the conversion of snow into glacial ice. (GeoMan)

Mid-oceanic ridge Elongated rise on the ocean floor where basalt periodically erupts, forming new oceanic crust; similar to continental rift zones (UCMP)

Mineral A naturally occurring, inorganic, crystalline solid with a definite internal structure and chemical composition. (GeoMan) A naturally occurring element or compound with a precise chemical formula and a regular internal lattice structure. Organic products are usually not included. (Geotech org)

Mineralogy The study of mineral composition, structure, appearance, stability, occurrence, and associations. (Geotech org)

Miogeosyncline A Geosyncline that is situated near a craton and receives chemical and well-sorted elastic sediments from the continent. (Geotech org)

Mohorovičić discontinuity The boundary between crust and mantle, marked by a rapid increase in seismic wave velocity to more than 8 kilometers per second. Depth: 5 to 45 kilometers. Abbreviated "Moho" or "M-discontinuity." (Geotech org)

Mohs scale of hardness An empirical, ascending scale of mineral hardness with talc as 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10. (Geotech org)

Moraine General term for material deposited beneath, along the sides, and/or at the terminus of a glacier. Also, what we get here in Oregon during the fall, winter, and spring. See also till. (GeoMan)

Mountain building see Orogeny.

Native Element occurs in nature and is not part of a compound. Examples are gold and silver. (J. Mellum, Wikiversity)

Oceanic crust The Earth's crust which is formed at mid-oceanic ridges, typically 5 to 10 kilometers thick with a density of 3.0 grams per cubic centimeter. (UCMP)

Oceanic trench Deep steep-sided depression in the ocean floor caused by the subduction of oceanic crust beneath either other oceanic crust or continental crust. (UCMP)

Orogen Linear to arcuate in plan, intensely deformed crustal belt associated with mountain building. compare craton . (S.M. Richardson)

Orogeny The tectonic process in which large areas are folded, thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with uplift and the formation of mountains. (Geowords glossary)

Outcrop Any place where bedrock is visible on the surface of the Earth. (UCMP)

Overturned fold An inclined fold in which one limb has been tilted beyond the vertical, so that the stratigraphic sequence within it is reversed. c.f. inclined fold. (S.M. Richardson)

Paleobathymetry The study of ocean depths and topography of the ocean floor in the geologic past. (USGS Paleontology glossary)

Paleoceanography The study of oceans in the geologic past, including its physical, chemical, biologic, and geologic aspects. (USGS Paleontology glossary)

Paleoclimate The average state or typical conditions of climate of a given period of time in the geologic past. (Geotech org, USGS Paleontology glossary)

Paleocurrent map A map of depositional currents that have been inferred from cross-bedding, ripples, or other sedimentary structures. (Geotech org)

Paleogeographic map A map showing the surface landforms and coastline of an area at some time in the geologic past. (Geotech org)

Paleomagnetism Refers to the study of the magnetic properties of rocks and minerals. This demonstrates to us that both the strength and direction of Earths magnetic field is not constant. Each rock and mineral tells its own story at a particular moment in time. (A. Atwal, Wikiversity)

Paleontology The science of ancient life, through examination of fossil remains and the fossil record. (MAK)

Paleosol A fossil soil or soil horizon. (MAK)

Paleowind A prevailing wind direction in an area, inferred from dune structure or the distribution of volcanic ash for one particular time in geologic history. (Geotech org)

Pangea (also spelt Pangaea): meaning "all the earth"; supercontinent that existed during the Permian and Triassic, and included most of the Earth's continental crust. During this time, terrestrial faunas were often quite uniform, as there were few geographic barriers, although there were distinct vegetation zones (biomes). Beginning in the Jurassic, Pangea divided into Laurasia in the north and Gondwana in the south.

Pegmatite A very coarse grained igneous rock like granite; commonly associated with large masses of plutonic rock. (M. MacFadzen, Wikiversity)

Period a unit or division of geological time, usually lasting several tens of millions of years, and hence intermediate in duration between era and epoch. By convention, each period is divided into two or more epochs. In terms of geological strata, rather than time, the word "system" is traditionally used, although this now seems to be falling out of favour, and only found in older books. (MAK)

Phanerozoic the most recent, and current, of the four eons of the geological timescale, the time of diverse and complex life, complex ecosystems, and an oxygen-rich atmosphere. Divided into Paleozoic, Mesozoic, and Cenozoic. The Phanerozoic begins with the start of the Cambrian period, and continues to today.

Plate A rigid segment of the Earth's lithosphere that moves horizontally and adjoins other plates along zones of seismic activity. Plates may include portions of both continents and ocean basins. (S.M. Richardson)

Plate tectonics The theory, proposed by Alfred Wegener, that the surface of Earth is made of a number of plates, which have moved throughout geological time to create the present-day positions of the continents. Plate tectonics explains the location of mountain building, as well as earthquakes and volcanoes. The rigid plates consist of continental and oceanic crust together with the upper mantle, which "float" on the semi-molten layer of the mantle beneath them, and move relative to each other across the planet. Six major plates (Eurasian, American, African, Pacific, Indian, and Antarctic) are recognized, together with a number of smaller ones. The plate margins coincide with zones of seismic and volcanic activity. (PBS evolution Glossary)

Proterozoic An eon of geologic time extending from about 3.8 to 2.5 billion years ago. It was preceded by the Archean and followed by the Paleozoic.

Sediment Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on the Earth's surface. Sediments can also result from chemical precipitation or secretion by organisms. (USGS Paleontology glossary)

Sedimentary basin A region of considerable extent (at least 10,000 km2) that is the site of accumulation of a large thickness of sediments. (Hugh Rance)

Sedimentary environment A geographically limited area where sediments are preserved; characterized by its landforms, climate, relative energy of water and wind currents, biological activity, and the relative abundance of various chemical substances. (Hugh Rance)

Sedimentary rock A rock formed by the accumulation and cementation of mineral grains transported by wind, water, or ice to the site of deposition or chemically precipitated at the depositional site (Geotech org), A rock that is the result of consolidation of sediments. (USGS Paleontology glossary)

Sedimentary structure Any structure of a sedimentary or weakly metamorphosed rock that was formed at the time of deposition; includes bedding, cross-bedding, graded bedding, ripples, scour marks, mudcracks. (Hugh Rance)

Shale a fine-grained sedimentary rock formed by the compaction of silt, clay, or sand that accumulates in deltas and on lake and ocean bottoms. (Glossary - Bristol University)

Spreading Ridge an elevated region where new crust material is being formed and where older crust moves away from the ridge. (B. Qaderi, Wikiversity)

Stratigraphy Branch of geology concerned with the formation, composition, ordering in time, and arrangement in space of sedimentary rocks. (USGS Paleontology glossary) The description, correlation, classification and dating of rock strata.

Superposition A statement of relative age in layered rocks: In a series of sedimentary rocks that has not been overturned, the topmost layer is always the youngest and the bottommost layer is always the oldest. (S.M. Richardson)

Syncline A fold that is convex downward, or that had such an attitude at some stage in its development. compare anticline. (S.M. Richardson)

Tethys during the time of Pangea (Permian and Triassic) this was the sea that separated the northern half (Laurasia) of the supercontinent from the southern (Gondwana). If Pangea can be imagined in the shape of a giant "pac-man", then the Tethys is the "mouth". During the Triassic especially, the borders of the Tethys were populated by unique animals, such as the walrus and turtle like placodonts. (MAK)

Terrane (microplate) A fragment of the lithosphere, smaller than a plate, that forms a portion of an accreted terrane margin . (S.M. Richardson)

Till (glacial) General term for material deposited by a glacier. Unstratified glacial drift consisting of rock fragments, clay, sand, gravel, and boulders that range from clay to boulder size and randomly arranged without bedding. See also moraine. (GeoMan, S.M. Richardson, UCMP)

Tsunami A great sea wave produced by a submarine earthquake, volcanic eruption, or large landslide. Commonly (but erroneously) called a "tidal wave," tsunamis can cause great damage due to flooding of low coastal areas. (GeoMan)

Tufa see travertine. (S.M. Richardson)

Tuff A general term for all consolidated pyroclastic rock. Not to be confused with tufa. (S.M. Richardson)

Ultramafic rock An igneous rock consisting dominantly of mafic minerals, containing less than 10 percent feldspar. Includes dunite, peridotite, amphibolite, and pyroxenite. (Geotech org)

Unconformity A buried erosion surface separating two rock masses. (S.M. Richardson)

Upper In geology, refers to the upper, and therefore younger or more recent strata. In the geological timescale, refers to the younger rocks of a period, especially when the period is divided into three; e.g. "Upper Jurassic". This usage is now generally replaced by the more chronological descriptive term Late. (MAK)

uniformitarianism The principle that applies to geology our assumption that the laws of nature are constant As originally used it meant that the processes operating to change the Earth in the present also operated in the past and at the same rate and intensity and produced changes similar to those we see today. The meaning has evolved and today the principle of uniformitarianism acknowledges that past processes, even if the same as today, may have operated at different rates and with different intensities than those of the present. The term "actualism" is sometimes used to designate this later meaning. (S.M. Richardson)

Volcanic Island Arcs When either oceanic crust subducts under oceanic crust (oceanic arc) or continental crust subducts under continental crust (continental arc). (Q.Janmohamed, Wikiversity)

Weathering The process by which Earth materials change when exposed to conditions at or near the Earth's surface and different from the ones under which they formed. compare decomposition, disintegration. (S.M. Richardson)

Xenolith fragments of country rock (detached by stoping) that sink into the magma. They are not fully absorbed by the magma and so remain as a mass. Some can be seen in igneous rocks. (Wikiversity)